Online-control cooling process for seamless steel tube for effectively refining grains and the method for manufacturing thereof

ABSTRACT

An online-control cooling process for seamless steel tube for effectively refining grains, comprising the following steps: when the temperature of a crude tube is higher than Ar3, evenly spraying water along the circumferential direction of the tube so as to continuously cool the tube to T1° C.˜T2° C., the cooling rate being controlled to be N1° C./s˜N2° C./s, wherein T1=810−360C−80(Mn+Cr)−37Ni−83Mo, T2=T1+115° C., N1=55-80×C, N2=168*(0.8−C), and C, Mn, Cr, Ni and Mo in the equations each represent the mass percentage of corresponding elements in the seamless steel tube; then, cooling to the room temperature at cooling rate no more than 10° C./s. Correspondingly, also provided are a method for manufacturing seamless steel tube for effectively refining grains, and a seamless steel tube. The online-control seamless steel tube cooling process does not require adding too many alloying elements, which is simple and can yield seamless steel tubes with good grain refinement and better toughness.

FIELD

The present invention relates to a controlled cooling process, in particular to an online control cooling process of a seamless steel tube.

BACKGROUND

In the prior art, due to product shape and manufacturing method limitations for hot-rolled seamless steel tubes, the product performance has long been improved only by addition of alloying elements and off-line heat treatment after rolling. Taking oil well tubes for example, tubes having a degree of 555 MPa (80 Ksi) or higher requires addition of more alloying elements in manufacturing, which significantly increases the manufacturing cost. Or, it can be made from conventional steel by off-line quenching heat treatment, wherein the so-called off-line quenching heat treatment means that hot-rolled seamless steel tubes are air-cooled to the room temperature after rolling, and be put into a tube bank firstly, then the pipes are heat-treated as needed. However, this method also complicates the process and increases the cost.

The performance of the steel is directly influenced by the grain size. Fine grain strengthening is the only strengthening mechanism that improves both strength and toughness of the steel at the same time. In general, the cooling rate of a hot steel tube (austenitic state) is accelerated by means of blowing or spraying water, which increases the degree of widercooling of austenite, promotes the nucleation of ferrite, and helps to improve grain refinement and strength.

Although those skilled in the art already know that on-line rapid cooling help the seamless steel tube obtain finer grain and better performance, the on-line rapid cooling is still not used in the prior art. This is because, cooling too fast will cause phase transition of bainite and martensite, although the strength of the seamless steel tube will increase significantly, rapid cooling often leads to great changes in material properties, such as decreasing in toughness and elongation, and increasing in yield ratio, etc. Such changes may not satisfy the requirements. On the other hand, the steel tube has a higher internal stress than that of sheet products due to its unique cross section, cooling too fast may lead to cracking and other problems.

Therefore, it is desired to obtain an online-control cooling process for seamless steel tube, which utilizes the waste heat after hot rolling of the steel pipe, wherein the online cooling process is controlled, grains are effectively refined and the toughness of seamless steel tube increases without non-equilibrium phase transition of bainite, martensite, etc.

Invention Contents

One of purposes of the present invention is to provide an online-control cooling process for effectively refining grain seamless steel tube. By using said process, the seamless steel tubes with good grain refinement can be obtained without adding large amount of alloying elements.

Based on the above invention purpose, the present invention provides an online-control cooling process for seamless steel tube for effectively refining grains, comprising the following steps:

when the temperature of tube is higher than Ar3, evenly spraying water along the circumferential direction of the tube so as to continuously cool the tube to temperature of T1° C.˜T2° C., the cooling rate being controlled to N1° C./s˜N2° C./s, wherein T1=810−360C−80(Mn+Cr)−37Ni−83Mo, T2=T1+115° C., N1=55−80×C, N2=168×(0.8−C), and C, Mn, Cr, Ni, and Mo in the equations each represents the mass percentage of corresponding elements of the seamless steel tube;

Then, cool the tube to the room temperature at a cooling rate no more than 10° C./s.

As already explained above, the method of on-line rapid cooling is not used to cool the steel pipe in the prior art, because this cooling method will cause the phase transition of hainite and martensitic, resulting in decreasing in toughness and elongation of the steel pipe. In addition, since the internal stress level of the seamless steel tube is much higher than those of offline re-heated austenitizing after the thermal deformation of the seamless steel tube, the seamless steel tube with online rapid cooling is likely to crack. In order to solve this technical problem, the inventors of the present invention conducted a lot of research and found that in order to make the grain significantly refined without the occurrence of phase transition of bainite or martensitic transformation, it is required to strictly control the quenching starting temperature, the final cooling temperature of quenching and the cooling rate, so as to coordinate with the element content of the steel effectively. Based on above, the inventors of the present invention propose said technical solution.

In this technical solution, the temperature of the tube needs to be higher than the Ar3 temperature, this is because some proeutectoid ferrite forms in the seamless steel tube if the online-control cooling process for seamless steel tube begins at a temperature below Ar3, which will deteriorate the grain refined effect and performance of the seamless steel tube.

In addition, the temperature of the continuous cooling of the tube is controlled from T1° C. to T2° C., wherein T1=T1=810−360C−80(Mn+Cr)−37Ni−83Mo and T2=T1+115° C. The inventors found that, a better implementation effect can be obtained when the final cooling temperature of the continuous cooling of the tube is controlled in the said range. When the final cooling temperature of the continuous cooling of the tube is higher than T2° C., the undercooling of the austenite is not enough, and the effect of the refining grains is not enough. When the final cooling temperature of the continuous cooling of the tube is lower than T1° C., the phase transition of bainite or martensite occurs and has an negative effect on the final performance of seamless steel tube. Therefore, in the said online-control cooling process for the seamless steel tube according to the present invention, the continuous cooling of the tube is controlled from T1° C. to T2° C.

Moreover, the inventors of the present invention also found that the seamless steel tube will obtain a better performance when the cooling rate is controlled from N1° C./s to N2° C./s, N1=55−80×C and N2=168×(0.8−C). When the cooling rate is lower than N1° C./s, subcooling of austenite is insufficient, on the other hand, when the cooling rate higher than N2° C./s, the steel tube is likely to crack. Therefore, in the online-control cooling process of the seamless steel tube according to the present invention, the cooling rate is controlled from N1° C./s to N2° C./s.

It should be noted that the Ar3 temperature is known to those skilled in the art or can be obtained under technical conditions. For example, it can be obtained by referring to a manual or by thermal simulation experiment.

In addition, it should be noted that, in the above equations, C, Mn, Cr, Ni and Mo each represents the mass percentage of corresponding elements of the seamless steel tube. That is, the numerical values of C, Mn, Cr, Ni and Mo substituted into the equations are the numerical values before the percent %. For example, in one embodiment where C is 0.17% by mass, the substituted value of C into the equations is 0.17, rather than 0.0017. The substitution of other elements has same meaning and is not further described.

It should also be noted that, the technical solution above which defines the above equations do not mean that the seamless steel tube must contain elements of C, Mn, Cr, Ni and Mo at the same time. The equations are general and can be applied to the seamless steel tube quenched by this method. Therefore, when one or more of the elements involved in the equations is not contained, zero should substitute into the equations.

In addition, in the present technical solution, grains are further refined by setting a step of air-cooling after rapid cooling. Since a high undercooling degree of the austenite is formed during the rapid cooling in the air-cooling step of seamless steel tube, the cooling rate for air-cooling cannot be too fast. When the cooling rate of air-cooling exceeds 10° C./s, it brings significant phase transition of bainite. Therefore, in this technical solution, the cooling rate in air cannot exceed 10° C./s.

Further, in the online-control cooling process for seamless steel tube according to the present invention, the total amount of alloying elements of the seamless steel tube is not more than 3% by mass, wherein alloying elements are at least one selected from C, Mn, Cr, Mo, Ni, Cu, V, Nb and Ti. If the alloying elements of the seamless steel tube exceed 3% by mass, the bainite/martensite phase can be obtained by air-cooling, to which said method cannot apply. In addition, the alloying element of the seamless steel tube in the present technical solution is not limited to C, Mn, Cr, Mo, Ni, Cu, V, Nb and Ti, and may be other alloying elements.

Further, in the online-control cooling process for seamless steel tube according to the present invention, the total amount of alloying elements of the seamless steel tube is 0.2% to 3% by mass.

The technical solution is particularly suitable for conventional carbon steel or low-alloy steel. By this process, seamless steel tube that meets performance requirements can be produced without adding excessive of alloying elements.

Accordingly, another purpose of the present invention is to provide a. method of manufacturing a seamless steel tube for effectively refining grains, comprising the steps of:

(1) manufacturing the Billet;

(2) forming the Billet into tube;

(3) cooling the tube by the online-control cooling process for seamless steel tube.

In the method for producing an effectively refined grain seamless steel tube according to the present invention, the implementation effect of effectively refining the grain is achieved by the online-control cooling process of the seamless steel tube described above. Compared with the prior art, the seamless steel tube can be austenitized without being reheated in the technical solution of the present invention, and the seamless steel tube has a better toughness by directly using online-control cooling process for the seamless steel tube.

It should be noted that, in step (1), the billet can be produced by casting the smelted molten steel into a round billet, or can be produced by pouring first and then forging or rolling the slab into the billet.

Further, in the manufacturing method for a seamless steel tube according to the present invention, in step (2), the billet is heated to 1100 to 1130° C. and maintained for 1 to 4 hours, followed by piercing, rolling, stretch reducing or sizing, so as to obtain the tube.

In addition, another purpose of the present invention is to provide a seamless steel tube which is prepared by the method said above for manufacturing seamless steel tube.

Further, in the seamless steel tube of the present invention, the grain size grade thereof is at least 7.5.

Further, in the seamless steel tube of the present invention, the microstructure thereof is mainly in form of pearlite and ferrite, and the phase ratio of the pearlite and ferrite is not less than 80%.

Further, in the seamless steel tube of the present invention, the microstructure thereof further contains bainite and/or cementite.

The online-control cooling process and the manufacturing method for the seamless steel tube for effectively refined grain according to the present invention have the following advantages and beneficial effects:

(1) The on-line control cooling process for the seamless steel tube according to the present invention can effectively refine the grains so that the grain size grade of the seamless steel tube obtained reaches at least 7.5.

(2) The on-line control cooling process and the manufacturing method for the seamless steel tube according to the present invention can effectively improve the toughness of the steel pipe and greatly reduce the amount of addition of the alloying elements at the same performance level.

(3) The on-line control of the cooling process and the manufacturing method for the seamless steel tube according to the present invention can avoid the cracking phenomenon of seamless steel tube which is unavoidable in the prior art and ensure the qualified rate of the product.

DETAILED DESCRIPTION

The online-control cooling process for the seamless steel tube for effectively refined grains according to the present invention will be further explained and described accompanying drawings and the specific Example as follow, and the this explanation and description shall not be deemed to limit to the technical solution of the present invention.

EXAMPLE A1-A7 AND COMPARATIVE EXAMPLE B1-B6

Seamless steel tubes in Example A1-A7 were manufactured according to the following steps:

(1) Manufacturing the Billet: smelting according to the mass percentage of each chemical element listed in Table 1, casting it into an ingot and forging the ingot into the Billet.

(2) forming the Billet into tube: the Billet is heated to 1100° C. to 1130° C. and maintained for 1 to 4 hours, followed by piercing, rolling, stretch reducing or sizing, so as to obtain the tube.

(3) using the online-control cooling process: when the temperature of tube is higher than Ar3, evenly spraying water along the circumferential direction of the tube so as to continuously cool the tube to temperature of T1° C.˜T2° C., the cooling rate being controlled to N1° C./s˜N2° C./s, wherein T1=810−360C−80(Mn+Cr)−37Ni−83Mo, T2=T1+115° C., N1=55−80×C, N2=168×(0.8−C), and C, Mn, Cr, Ni, and Mo in the equations each represents the mass percentage of corresponding elements of the seamless steel tube; then, cooling to the room temperature at a cooling rate no more than 10° C./s.

In order to demonstrate the implementation effect of the online-control cooling process of the present invention, the process steps of manufacturing the billet and the tube for Comparative Example B1-B6 are the same as that for Example of the invention, whereas the process parameters of control cooling process for Comparative Example B1-B6 are outside the protection scope of the present technical solution.

Table 1 lists each mass percentage of the chemical elements of the seamless steel tubes of Example A1 to A7 and Comparative Example B1 to B6.

TABLE 1 (by wt %, the margin is Fe and other unavoidable impurity elements) Steel No. model C Mn Cr Mo Ni A1 16Mn 0.17 1.65 — — — A2 20# 0.2 0.5 — — — A3 20# 0.2 0.5 — — — A4 20# 0.2 0.5 — — — A5 30Mn2 0.3 1.55 — — — A6 20CrNi 0.2 0.55 0.9 — 1.05 A7 15NiMo 0.15 0.6 0.2 0.60 B1 16Mn 0.17 1.65 — — — B2 20# 0.2 0.5 — — — B3 20# 0.2 0.5 — — — B4 20# 0.2 0.5 — — — B5 20# 0.2 0.5 — — — B6 20# 0.2 0.5 — — —

Table 2 lists the specific process parameters for the methods for manufacturing seamless steel tube of Example A1-A7 and Comparative Example B1-B6.

TABLE 2 Quenching Final Heating heating Ar3 starting cooling temperature time temperature temperature T1 T2 temperature N1 N2 Cooling rate Cooling rate No. (° C.) (h) (° C.) (° C.) (° C.) (° C.) (° C.) (° C./s) (° C./s) (° C./s) in air/° C./s A1 1280 2.8 835 930 616.8 731.8 654 41.4 105.84 61 3 A2 1140 3.5 865 920 698 813 724 39 100.8 42 5 A3 1260 2.5 865 920 698 813 735 39 100.8 73 1.5 A4 1150 1.4 865 970 698 813 728 39 100.8 55 1.8 A5 1250 2.5 721 780 578 693 660 31 84 38 8 A6 1200 2 790 940 583.15 698.15 625 39 100.8 52 6 A7 1240 2.5 750 900 669.2 784.2 694 43 109.2 75 4.6 B1 1250 2 835

616.8 731.8 628 41.4 105.84 48 2.5 B2 1250 2 865 860 698 813 712 39 100.8

4 B3 1250 2 865 940 698 813

39 100.8 46 5 B4 1250 2 865 900 698 813 750 39 100.8

8 B5 1250 2 865 920 698 813

39 100.8 42 5 B6 1250 2 865 920 698 813 716 39 100.8 42

Various performance tests were conducted on the seamless steel tubes of Example A1-A7 and Comparative Example B1-B6, and the results are shown in Table 3. Wherein the yield strength data are average value obtained according to the API standard after the seamless steel tube of Example A1-A7 and the seamless steel tube of Comparative Example B1-B6 are processed into API arc-shaped samples. The impact sample was test by the standard impact sample of the seamless steel tube of Example A1-A7 and Comparative Example B1 to B6 processed into 10 mm*10 mm*55 mm size, V-notch at 0° C. In addition, the hardness after quenching cooling of each Example and Comparative Example was measured by a Rockwell hardness test. The grain size was measured according to GB/T6394 standard after sampling, and the phase ratio was measured by the metallographic method.

TABLE 3 Performance data for each Example and each Comparative Example Impact energy Phase (full ratio Yield size Of Strength sample) Pearlite + Crack/ Rp0.2 at 0° C. Grain ferrite yes No. (MPa) (J) size (%) or no A1 453 198 7.5 85 no A2 336 147 8 92 no A3 342 152 8 87 no A4 340 123 7.5 94 no A5 594 98 8 90 no A6 582 168 8.5 88 no A7 378 172 8.5 95 no B1 368 144 6 89 no B2 253 97 6.5 92 no B3 262 108 6.5 87 no B4 — — — — yes B5 428 16 6.5 24 no B6 359 32 5.5 31 no

As can be seen from Table 3, the yield strength of the seamless steel tubes for all Example A1-A7 is ≥336 MPa, the impact energy at 0° C. thereof is higher than 98J, and the grain size grade is higher than 7.5, and the phase ratio of the pearlite and ferrite in the microstructure of which is ≥80%.

As can be seen from Table 2 and Table 1, the component ratios of the chemical elements for all Example and Comparative Example have no difference, but the method for manufacturing of the Example and Comparative Example are significantly different. Therefore, the performance of the seamless tube of Example A1-A7 is superior to that of Comparative Example B1-B6 overall. in addition, as can be seen from Table 2 and Table 3, the quenching starting temperature of Comparative Example B1 is lower than the Ar3 temperature so that the steel of Comparative Example B1 precipitates proeutectoid ferrite, reducing its hardness after quenching and affecting the strength of seamless steel tube also. The cooling rate of Comparative Example B2 is lower than the cooling rate range defined in the present technical solution, thus the desired microstructure could not be obtained, which will affect the performance. The final cooling temperature of Comparative Example B3 was higher than the T2° C. of the present invention, thus the desired microstructure of seamless steel tube could not be obtained in Comparative Example B3, which will affect the performance. In addition, the cooling rate of Comparative Example B4 is higher than the cooling rate range defined in the present technical solution, so that the steel tube cracked, the hardness is insufficient. The final cooling temperature of Comparative Example 95 is lower than T1° C. as defined in the present technical solution, and the cooling rate in air of Comparative Example B6 is higher than the cooling rate range defined in the present technical solution, which results in a significant phase transition of bainite in Comparative Example B5 and Comparative Example B6, and lack of toughness.

It is to be noted that the above Example are only a specific embodiments of the present invention. Apparently, the invention is not limited to the above embodiments, and there are may be many similar variations. A person skilled in the art can directly derive or associate all the variations from the content disclosed by the invention, all of which shall be covered by the protection scope of the invention. 

1. An online-control cooling process for seamless steel tube for effectively refining grains, comprising the following steps: spraying water evenly along the circumferential direction of the tube when the temperature of tube is higher than Ar3 so as to continuously cool the tube to temperature of T1° C.˜T2° C., and controlling; the cooling rate to N1° C./s to N2° C./s, wherein T1=810−360C−80(Mn+Cr)−37Ni−83Mo, T2=T1+115° C., N1=55−80×C, and N2=168×(0.8−C), and wherein C, Mn, Cr, Ni, and Mo in the equations each represents the mass percentage of corresponding elements of the seamless steel tube; and cooling to room temperature at a cooling rate no more than 10° C./s.
 2. The online-control cooling process for seamless steel tube according to claim 1, wherein the total amount of alloying elements of the seamless steel tube is not more than 3% by mass, said alloying elements being at least one selected from the group consisting of C, Mn, Cr, Mo, Ni, Cu, V, Nb and Ti.
 3. The online-control cooling process for seamless steel tube according to claim wherein the total amount of alloying elements of the seamless steel tube is 0.2% to 3% by mass.
 4. A method for manufacturing seamless steel tube for effectively refining grains, comprising the following steps: (1) manufacturing a billet; (2) forming the billet into a tube; (3) cooling the tube by the online-control cooling process for seamless steel tube according to claim
 1. 5. The method for manufacturing seamless steel tube according to claim 4, wherein the obtained seamless steel tube has a grain size grade of at least 7.5.
 6. The method for manufacturing seamless steel tube according to claim 4, wherein in step (2), the billet is heated to 1100° C. to 1130° C. and maintained for 1 to 4 hours, and further comprises piercing, rolling, stretch reducing or sizing, so as to obtain the tube.
 7. A seamless steel tube, which is prepared by the method for manufacturing seamless steel tube according to claim
 4. 8. The seamless steel tube according to claim 7, wherein the microstructure of steel is mainly in form of pearlite and ferrite, and the phase ratio of the pearlite and ferrite is not less than 80%.
 9. The seamless steel tube according to claim 8, wherein the microstructure further contains bainite and/or cementite.
 10. A seamless steel tube, which is prepare e od for manufacturing seamless steel tube according to claim
 5. 11. The seamless steel tube according to claim 10, wherein the microstructure of steel is mainly in form of pearlite and ferrite, and the phase ratio of the pearlite and ferrite is not less than 80%.
 12. The seamless steel tube according to claim 11, wherein the microstructure further contains bainite and/or cementite
 13. A seamless steel tube, which is prepared by the method for manufacturing seamless steel tube according to claim
 6. 14. The seamless steel tube according to claim 13, wherein the microstructure of steel is mainly in form of pearlite and ferrite, and the phase ratio of the pearlite and ferrite is not less than 80%.
 15. The seamless steel tube according to claim 14, wherein the microstructure further contains bainite and/or cementite 